f2fs: compress: fix to avoid redundant compress extension

With below script, redundant compress extension will be parsed and added
by parse_options(), because parse_options() doesn't check whether the
extension is existed or not, fix it.

1. mount -t f2fs -o compress_extension=so /dev/vdb /mnt/f2fs
2. mount -t f2fs -o remount,compress_extension=so /mnt/f2fs
3. mount|grep f2fs

/dev/vdb on /mnt/f2fs type f2fs (...,compress_extension=so,compress_extension=so,...)

Fixes: 4c8ff709 ("f2fs: support data compression")
Fixes: 151b1982

 ("f2fs: compress: add nocompress extensions support")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>